In a laser machining using an optical fiber, when an irradiated laser beam is reflected by a machining target object and re-entered into a cladding of an optical fiber as a returning light, there is a possibility for burning out a coating layer and the like of an optical fiber. Then, in a case where an amount of light of that returning light is large, there is also a concern for resulting in a melting or a disconnection of an optical fiber itself. For this reason, a technique for detecting a returning light and controlling an operation of a laser oscillator according to its amount of light has been proposed. In the patent document 1, it is described that, in an optical fiber with a triple structure of a core, a cladding and an outer core, a returning light leakage unit for leaking a returning light to an external is provided, a sensor for detecting a leakage light is provided at a position corresponding to a light leakage unit in a vicinity of a side face of the optical fiber, and an oscillation of the laser oscillator is stopped when the sensor detects the leakage light.